The invention relates to a support device provided with a first part, a second part, a gas spring with a pressure chamber for supporting the second part relative to the first part parallel to a support direction, and a gas supply for the supply of gas to the pressure chamber.
The invention also relates to a lithographic device provided with a radiation source, a mask holder, a focusing unit with a main axis, a positioning device with a substrate holder which is displaceable relative to the focusing unit parallel to an X-direction which is perpendicular to the main axis and parallel to a Y-direction which is perpendicular to the X-direction and perpendicular to the main axis, a frame which supports at least the focusing unit parallel to a support direction, and a base which supports the frame by means of at least three support devices which each exert a supporting force on the frame which is directed parallel to the support direction.
A support device and a lithographic device of the kinds mentioned in the opening paragraphs are known from WO-A-96/38766, to which U.S. Pat. No. 5,844,664 corresponds. The known lithographic device is used in the manufacture of integrated semiconductor circuits by means of an optical lithographic process. The radiation source in the known lithographic device is a light source, and the focusing unit is an optical lens system by means of which a sub-pattern of an integrated semiconductor circuit, which sub-pattern is present on a mask which can be placed on the mask holder, is imaged on a reduced scale on a semiconductor substrate which can be placed on the substrate holder. Such a semiconductor substrate comprises a large number of fields on which identical semiconductor circuits are provided. The individual fields are for this purpose consecutively exposed through the mask, a next field of the semiconductor substrate being brought into position relative to the focusing unit by the positioning device between two consecutive exposure steps each time. This process is repeated a number of times, each time with another mask with a different sub-pattern, so that integrated semiconductor circuits with comparatively complicated structures can be manufactured. Since such structures have detail dimensions which lie in the submicron range, the sub-patterns present on the consecutive masks must be imaged on the fields of the semiconductor substrate with an accuracy which lies in the submicron range. The substrate holder and the mask holder should accordingly occupy accurate positions relative to the focusing unit during operation.
The frame of the known lithographic device is supported in a vertical direction by means of three support devices of the known kind, the first parts of the support devices being fastened to the base of the lithographic device, and the second parts of the support devices being fastened to the frame. The frame of the known lithographic device supports not only the focusing unit but also the mask holder and the substrate holder in the vertical direction, while the base can be placed on a floor. Since the substrate holder and the mask holder have to occupy accurate positions relative to the focusing unit during operation, vibrations of the frame are to be prevented as much as possible during operation. Such vibrations of the frame may be caused by vibrations in the base which arise, for example, from floor vibrations. Vibrations in the base of the known lithographic device are in addition caused by reaction forces of the positioning device of the substrate holder and reaction forces of a further positioning device by means of which the mask holder is displaceable relative to the focusing unit. The three support devices accordingly serve not only for supporting the frame in the vertical direction, but also for preventing a transmission of vibrations from the base to the frame.
The pressure chamber of the gas spring of the known support device is bounded by a cylindrical cup and by an annular membrane by means of which the cup is suspended in the pressure chamber of the gas spring. The cup is fastened to the second part by means of three comparatively thin tension rods which are positioned in the cup and which extend parallel to the support direction. To prevent a transmission of vibrations from the base to the frame as much as possible, a mass spring system formed by the support devices and the frame together with the components of the lithographic device supported by the frame should have natural frequencies which are as low as possible parallel to the support direction as well as perpendicular to the support direction. The pressure chamber of the gas spring of the known support device has a comparatively large volume for providing a natural frequency of said mass spring system which is as low as possible parallel to the support direction. The tension rods mentioned above have a comparatively great length in order to provide a natural frequency of said mass spring system which is as low as possible in directions perpendicular to the support direction.
The gas supply of the known support device is connected to a valve which is mounted in a wall of the pressure chamber. Since an average gas pressure prevailing in the pressure chamber will fall during operation as a result of inter alia gas leakage through the membrane, gas is to be supplied to the pressure chamber via the gas supply at regular intervals. During the supply of gas, pressure fluctuations present in the gas supply are transmitted to the pressure chamber. Such pressure fluctuations are caused, for example, by a compressor connected to the gas supply. The pressure fluctuations in the pressure chamber cause mechanical vibrations of the second part of the support device, which are transmitted to the frame of the lithographic device. The accuracy of the lithographic device is adversely affected thereby. To prevent such an undesirable adverse effect on the accuracy of the known lithographic device, this lithographic device must be stopped while gas is being supplied to the pressure chambers of the support devices, which adversely affects the production output of the lithographic device.